In a receiver used in communication systems, an interference signal (i.e., spurious signal) that is different from the transmitted signal may be mixed in the received signal. The spurious signal may emanate from another equipment, or may emanate from another signal source (e.g., clock system) situated in the receiver. The fact that quality of the received signal degrades due to the spurious signal may necessitate reduction of the effect of a spurious signal. A spurious signal is often an unmodulated wave signal or a narrowband modulated signal. Once the frequency position of the spurious signal is detected, the spurious signal may be reduced by suppressing the signal component at the detected frequency position.
A PLL (phase locked loop) circuit may lock to an unmodulated or narrowband modulated wave component mixed in a received signal, thereby detecting the frequency position of a spurious signal. A signal received by an antenna is converted into a received baseband digital signal by an RF circuit and an ADC (i.e., analog-to-digital converter). In the case of the position of the spurious frequency being roughly known, a PLL circuit is oscillated at this frequency position to supply the generated sinusoidal wave signal to a rotor. The rotor shifts the entirety of frequencies of the received digital signal such that the spurious frequency is situated at around 0 Hz, with the PLL circuit locking to the spurious frequency. The shifted received digital signal is supplied to a loop filter, which then generates a direct-current offset component responsive to the magnitude of the spurious signal. Subtraction of the direct-current offset component from the frequency-shifted received digital signal prior to being supplied to the loop filter serves to remove the spurious component. The received digital signal from which the spurious component is removed is input into a reverse rotor, which shifts the entirety of frequencies in an opposite direction to bring them back to the original frequency band. Decoding in accordance with the employed communication system is then performed with respect to the received digital signal.
In the case of the spurious frequency being not known, a frequency sweeper controls the NCO (i.e., numerically controlled oscillator) of the PLL circuit to gradually change the oscillating frequency. The sinusoidal wave signal generated by the NCO is supplied to the rotor and the reverse rotor. When the frequency specified by the frequency sweeper is situated near the spurious frequency, the loop filter of the PLL circuit outputs a direct-current offset component. The NCO is oscillated in response to the output of the loop filter, so that the PLL circuit locks to the spurious signal. As a result, the NCO can continuously supply a sinusoidal wave signal synchronized with the frequency and phase of the spurious signal to the rotor and the reverse rotor.
In the above-noted configuration, the signal waves are subjected to the frequency shift and reverse frequency shift that are provided by the rotor and the reverse rotor, respectively. Such shifts are made by using a limited number of bits representing the digital signals, resulting in a slight degradation of the signal waves due to the shift processes. When there is only one spurious wave, one frequency shift operation and one reverse frequency shift operation are all that is necessary. Degradation of signal waves in this case thus has only a small effect on the quality of received signals. When there are a plurality of spurious waves, however, frequency shift operations and reverse frequency shift operations as many as the number of spurious waves are performed. In such a case, the degradation of signal waves can no longer be ignored, resulting in degradation in the quality of received signals.    [Patent Document 1] Japanese Laid-open Patent Publication No. 2009-188602    [Patent Document 2] Japanese Laid-open Patent Publication No. 2010-226512